I'd pick two of those 20s and swap them for a 20-50-50-20 triplex breaker
Since your panel is listed and labeled to accept tandem/"double stuff" breakers in all positions, and looks to be uniformly configured with 20A breakers, we can take two adjacent 20A breakers in the panel and switch them out for a MP22050CT (if you can find it) or Q22050CT (if you can't find the Murray part) triplex breaker, then land the two existing 20A circuits you replaced on the two outer poles. You can then install your Type 2 SPD, landing the hots on the two 50A inner poles of the triplex breaker, the neutral on the right-hand neutral bar, and the ground on the left-hand ground bar. (Note that you'll need to use a SPD that has separate neutral and ground wires coming out; some SPDs only provide two hots and a neutral and thus can't be used on a subpanel.)
And I'd clean up those double taps while I was at it
While I was in here, I'd clean up those two double taps on the bottom-left and bottom-right breakers, as all that takes is a couple of pigtails of 12AWG wire and wirenuts suited for 3 12AWG wires. Take each double-tap out of the breaker terminal screw, nut a pigtail to the twisted ends of the two wires, and land that pigtail on the breaker terminal screw the double-tapped wires once went to. Repeat for the other breaker, and once that and the SPD install are done, you can button things up, turn the main breaker back on, and call it a day!